Optimization: Deliberate u8 overflow for universe size 256x256x256x6

[craigmayhew]

Currently we have an if statement for every rule, checking if we are at the edge of the universe. This could be avoided if we have a deliberate overflow without an index out of range error. This would be possible for a universe of size 256x256x256 using a u8.

This is twice as big as our performant universes that top out at about 140^3 and so may need other performance improvements first before it is possible.